Add CoT execution trace to the SSE stream: status events for stage transitions, enriched tool_start with input_summary, tool_end with output_summary and duration_ms, and tool_error for failed tool calls. Helper functions _sse, _summarize_input, _summarize_output provide human-readable summaries for each tool type. Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>